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There are two ways:
If you want specify baud rate and other parameters use TAPI functions and particualary OpenLine. You have to specify that line you want to open is not a modem. Tor more information and samples see comm sample in Viaul C++

Second way is much simpler. You have to open file (Use CreateFile function) instead of file name use "COM1","COM2" or port name you want to open.

After this you would be able to Read/Write using standart WIN32 API functions ReadFile/WriteFile. To use overlaped i/o use ReadFileEx/WriteFileEx